1. Field of the Invention
The present invention relates to a CMOS operational amplifier circuit, and more specifically relates a CMOS operational amplifier circuit which can use a broad input and output voltage range under a low voltage drive, permits a highly accurate amplification and is particularly suitable for use as a comparator circuit.
2. Background Art
A CMOS operational amplifier circuit is used in a system IC in which analogue signals and digital signals exist in a mixed manner and is suitable for when used as an amplifier circuit or a comparator circuit of a microscopic current in an order of xcexcA.
This sort of circuit usually includes as its input stage a differential amplifier having a pair of P channel MOS transistors or a pair of N channel MOS transistors and is constituted so as to transfer an output current from the differential amplifier to an output side through a current mirror circuit. However, in this sort of circuit, since there is a dead zone corresponding to a source-gate threshold voltage in the MOS transistors with respect to the same phase input, an input and output voltage in the range from the ground GND to a power source voltage VDD can not be fully used, and the dynamic range of the input and output voltage is limited.
As a measure for resolving the above problem, JP-A-3-62712 (1991) discloses a xe2x80x9cCMOS operational amplifier circuitxe2x80x9d in which a differential amplifier of P channel MOS transistors and another differential amplifier of N channel MOS transistors are provided at the input stage thereof.
In the disclosed circuit the differential amplifier of P channel MOS transistors and the differential amplifier of N channel MOS transistors provided as the input stage are driven at the same time by an input signal and their respective output currents are synthesized by a current mirror circuit. Thereby, in the disclosed circuit when one of the differential amplifiers of P channel MOS transistor and N channel MOS transistors is moved into its dead zone, an output of the other differential amplifier is obtained, thereby, an output substantially eliminating a dead zone is provided and a broad input and output voltage dynamic range from the ground GND to the power source voltage VDD is ensured.
With such CMOS operational amplifier circuit in which the differential amplifiers of P channel transistors and N channel transistors are provided and are driven at the same time, when the differential amplifier of P channel MOS transistor is placed in its dead zone, only the differential amplifier of N channel MOS transistors is operated, on the other hand, when the differential amplifier of N channel MOS transistors is placed in its dead zone, only the differential amplifier of P channel MOS transistors is operated. In the region excluding both dead zones both differential amplifiers are operated.
Therefore, when assuming, for example, that the power source voltage VDD is 1.8V, the dead zone voltage of N channel and P channel MOS transistors with respect to an input signal is 0.7V, a transconductance Gm of the differential amplifier of N channel MOS transistors is GmN and a transconductance Gm of the differential amplifier of P channel MOS transistors is GmP, a transconductance Gm of the CMOS operational amplifier circuit is given as Gm=GmP when the input signal voltage is in a range of 0xcx9c0.7V, when the input signal voltage is in a range of 0.7xcx9c1.1V, the transconductance Gm thereof is given as Gm=GmN+GmP and when the input signal voltage is in a range of 1.1Vxcx9c1.8V, the transconductance Gm thereof is given as Gm=GmN, thus, the transconductance Gm thereof varies depending on the input signal voltage. As a result, the gain bandwidth product (GB product) varies.
In such CMOS operational amplifier circuit the differential amplifiers which are operative differ depending on the input signal voltage and since the differential amplifiers are separately operated in the three operation ranges, and the GB product varies largely depending on the input signal voltage. As a result, selection of an optimum phase compensation capacitor is made difficult. Because of the difficulty of selecting such phase compensation capacitor, a problem is caused that the operational amplifier circuit is likely to be oscillated. Further, in the CMOS operational amplifier circuit having the above structure, since large steps with regard to the transconductance Gm before and after both dead zones are caused under a low power source voltage, another problem arises that a highly accurate amplification is prevented.
The present invention is intended to solve such conventional art problems and an object of the present invention is to provide a CMOS operational amplifier circuit which is suitable for a low voltage drive, can use a broad input and output voltage range and permits amplification with high accuracy.
A structure of a CMOS operational amplifier circuit according to a first aspect of the present invention which achieves the above object comprises a first differential amplifier having differential MOS transistors of one of P channel and N channel which receive an input signal and a second differential amplifier having differential MOS transistors of the other of P channel and N channel which receive the input signal and generates an output signal depending on the input signal in response to an output of these differential amplifiers, further comprises a current mirror circuit which receives respective current output signals of the first and second differential amplifiers and generates a current output signal depending on the current values of the respective current output signals, an output circuit which receives the output signal of the current mirror circuit and generates an output signal depending on the input signal and a change-over circuit for causing operation change-over which causes the first differential amplifier to stop operation when the first differential amplifier is entering or has been entered into a dead zone operation region with respect to the input signal and causes the second differential amplifier to perform operation.
Further, the structure of a CMOS operational amplifier circuit according to a second aspect of the present invention comprises a first bias current producing circuit which causes to flow a first current having the value corresponding to a bias current value of a differential transistor in the first differential amplifier and a second bias current producing circuit which causes to flow a second current having the value corresponding to a bias current value of a differential transistor in the second differential amplifier, wherein when the bias current flowing through the differential transistor in the first differential amplifier is caused to flow through or to be sinked in the current mirror circuit and when the bias current flowing through the differential transistor in the second differential amplifier is caused to flow through or to be sinked into the current mirror circuit, the current output signals of the first and second differential amplifiers are inputted, and the change-over circuit causes the first differential amplifier to operate as well as to flow the second current value from the second bias current producing circuit to the current mirror circuit or to sink the same from the current mirror circuit and when the first differential amplifier is entering or has entered into the dead zone operation region with respect to the input signal, the change-over circuit causes to stop the first differential amplifier to operate and the second current value from the second bias current producing circuit to flow, and causes to operate the second differential amplifier as well as causes to flow the first current value from the first bias current producing circuit to the current mirror circuit or causes to sink the same from the current mirror circuit.
Now, according to the first aspect of the present invention, through the provision of the change-over circuit, the first differential amplifier is caused to operate, and when the first differential amplifier is entering or has entered into the dead zone operation region with respect to the input signal, the operation of the first differential amplifier is stopped and the second differential amplifier is caused to be operated. Thereby, the transconductance Gm of the CMOS operational amplifier circuit assumes either one of that for the first and second differential amplifiers and variation of Gm with respect to the input signal is suppressed. Further, it is easy to design the first and second differential amplifier to have substantially the same Gm.
Therefore, variation of Gm with respect to the voltage variation of the input signal of thus constituted CMOS operational amplifier circuit is very small.
Further, according to the second aspect of the present invention, through the provision of the first and second bias current producing circuits which produce the respective bias currents for the differential transistors in the first and second differential amplifiers, since when one of the differential amplifiers is operating, the bias current of the other differential amplifier is supplied from the concerned bias current producing circuit to the current mirror circuit or a bias current is caused to flow out from a power source which flows the bias current to the current mirror circuit to the bias current producing circuit, even if an operation change-over between the first and second differential amplifiers is performed, the bias current in the current mirror circuit is kept to be hardly varied. As a result, the transconductance Gm of the current mirror circuit which receives the current outputs of the first and second differential amplifiers is substantially kept constant.
Accordingly, variation of the transconductance Gm of the CMOS operational amplifier circuit can be further suppressed.
As a result, a CMOS operational amplifier circuit can be easily realized which is suitable for a low voltage drive, can use a broad input and output voltage range and permits amplification with high accuracy.